The invention relates to a method for beamforming in a radio communications system having a base station whose associated antenna device has a number of antenna elements, so that spatial resolution is possible in the beamforming.
In radio communications systems, messages (speech, picture information or other data) are transmitted via transmission channels by electromagnetic waves (radio interface). The transmission takes place both in the downlink direction from the base station to the subscriber station and in the uplink direction from the subscriber station to the base station.
Signals which are transmitted using electromagnetic waves are subject, inter alia, to disturbances due to interference during their propagation in a propagation medium. Disturbances caused by noise may be caused, inter alia, by noise in the input state of the receiver. Due to diffraction and reflections, signal components pass over different propagation paths. Firstly, this means that a signal can arrive at the receiver more than once, in each case from different directions, with different delays, attenuations and phase angles, and, secondly, components of the received signal may be superimposed coherently with changing phase relationships in the receiver, leading to cancellation effects on a short-term timescale (fast fading) there.
DE 197 12 549 A1 discloses the use of intelligent antennas (smart antennas), that is to antenna arrangements having a number of antenna elements, in order to increase the transmission capacity in the uplink direction. These allow the antenna gain to be deliberately aligned in a direction from which the uplink signal is coming.
Various methods for spatial signal separation for the uplink and downlink directions are known from A. J. Paulraj, C. B. Papadias, “Space-time processing for wireless communications”, IEEE Signal Processing Magazine, November 1997, pages 49–83.
Particular difficulties occur in the downlink direction, that is to say from the base station to the subscriber station, since the beamforming has to be carried out before the transmitted signals are influenced by the radio channel. An algorithm for beamforming in the downlink direction is known from R, Schmalenberger, J. J. Blanz, “A comparison of two different algorithms for multi antenna C/I balancing”, Proc. 2nd European Personal Mobile Communications Conference (EPMCC), Bonn, Germany, September 1997, pages 483–490, which is based on a direct propagation path (visual link) between the base stations and the subscriber stations and iterative calculation of beamforming vectors. The entire, complicated, iterative calculation must be repeated whenever the characteristics of the transmission channel change.
DE 198 03 188 A discloses a method in which a spatial covariance matrix is defined for a link from a base station to a subscriber station. An eigen vector is calculated in the base station from the covariance matrix, and is used as a beamforming vector for that link. The transmission signals for the link are weighted with the beamforming vector and are supplied to antenna elements for emission. Intracell interference is not included in the beamforming process owing to the use of joint detection, for example in the terminals, and any corruption of the received signals by intercell interference is negligible.
In an environment with multipath propagation, this method clearly determines a propagation path with good transmission characteristics and concentrates the transmission power of the base station physically on this propagation path. However, using this approach, it is impossible to prevent the possibility of interference on this transmission path leading to signal cancelation, and hence to interruptions in the transmission, in the short term.
The recommendations from the 3GPP (3rd Generation Partnership Project, http://ww.3gpp.org) therefore provide methods in which the subscriber station estimates a short term channel impulse response hm for the channel from the m-th antenna element to the subscriber station, and calculates weighting factors wm which intended to be used for weighting the transmission signal before it is transmitted by the m-th antenna element. Corresponding concepts are also dealt with in M. Raitola, A. Hottinen and R. Wichmann, “Transmission diversity in wideband CDMA”, which appeared in the Proceedings of the 49th IEEE Vehicular Technology Conf. Spring (VTC 99 Spring), pages 1545–1549, Houston, Tex. 1999.
One serious problem with this procedure is that the vector of the weighting factors which is estimated by the subscriber station must be transmitted to the base station and that, in accordance with the Recommendations from the 3GPP, only a narrow bandwidth of 1 bit per time slot (slot) is available for this purpose. The vectors can thus be transmitted in only a coarsely quantized form. If the channel changes quickly and the weights need to be updated from one time slot to the next, only two different relative phase angles of the antenna elements can be set. If the channel is changing relatively slowly and, for example, four time slots are available for transmission of the vector, 16 different values of the vector can then be represented.
However, the known concepts reach their limits when the number of antenna elements at the base station is greater than 2, since the bandwidth which is required for transmission of the vector increases with its number of components, that is to say with the number of antenna elements. This means that, although a large number of antenna elements would be desirable on the one hand in order to make it possible to align the transmission beam as accurately as possible, the limited available bandwidth on the other hand means that the weighting vector cannot be updated sufficiently often as would be necessary for matching to fast fading.